Coated steel is a steel that has an extra finish applied to the natural steel surface. The extra coating or finish applied can be tin, chrome, zinc, paint or plastic. The application of the coating is done in order to protect the surface of the steel from oxidation and it is also used to enhance the appearance of the material. Coated steel can be divided into several different categories. These include hot-dip galvanized, galvannealed, galvalume, galfan, electrolytic zinc (electro-galvanized) and pre-painted steel.
Galvanization refers the process of coating steel with zinc. Galvanizing was named after its founder, Italian scientist Luigi Galvani. Galvanization was originally the process of electrical shock. The process was discovered as Galvani touched a frog’s leg with forceps and saw the muscles twitch. The use of the word galvanize in today’s terms refers mainly to the chemical process of hot dip galvanizing. In the process steel or iron is coated with zinc. The zinc reduces the chances of corrosion.
Hot-dip galvanized steel is carbon steel that is coated with a thin zinc layer on both sides using a hot-dip continuous line. The coated zinc layer is tightly bound to the steel in a molten bath of zinc at a temperature of around 460 Celsius. Galvanized steel can be used in a wide variety of applications where rust resistance is needed. One of the big identifying characteristics of galvanized steel is the spangle or visible crystallization pattern on the steel’s surface .
Galvannealed steel is carbon steel that has been coated with a zinc coating on both sides using a hot-dip continuous line. When the steel leaves the molten zinc bath the molten zinc coating is sent to an in-line heat treatment. This converts it into an iron-zinc alloy as the iron is diffused from the steel into the coating. Galvannealed steel is different from hot-dip galvanized steel as it has no visible spangle and it appears matte compared to the highly metallic appearance of galvanized steel. The reddish-orange appearance on of the surface of the steel is because of the iron spread over the coated layer particularly when the iron content ranges 8 to 11 percent.
Galvalume coated steel is carbon steel that is coated with an aluminum-zinc coating using a hot-dip continuous line. The coating mixture consists of 45 percent zinc and 55 percent aluminum. A small amount of silicon is also added to the mixture and this helps with the adhesion of the coating. The corrosion resistance is much better than that of hot-dip galvanized steel. This makes it very popular in environments with prolonged exposure to corrosion.
Galfan steel is carbon steel that has been coated with a mixture of aluminum and zinc on both sides using a hot-dip continuous line. The composition of the coating layer is 95 percent zinc and 5 percent aluminum. It is more corrosion resistant than normal hot-hip galvanized and generally cheaper to produce than galvalume steel thus making it the preferred material for use in areas that needs long term resistance to atmospheric corrosion.
Electro-galvanized steel is carbon steel that is coated with pure zinc coating by applying an electric current. This causes the positively charged zinc anodes to attach to the negatively charged steel. This process helps to smooth the surface even after painting. The final product has a similar appearance to cold rolled steel but it is darker and it has similar mechanical properties to the base material of cold rolled coil.
Pre-painted steel is a steel that has received a layer of paint after a treatment. In many cases the pre-painted steel also has a layer of primer applied. The paint can be applied to uncoated steel or steel that has a metallic coating. In order to achieve maximum corrosion resistance a metallic coated steel is used.
These include galvalume, galfan, galvannealed and galvanized steel. Pre-painted steel technology has been developing fast over the past twenty years. Advances in fade, chalk resistance, flexibility, hardness in painting materials and many more has widened the use of the pre-painted steel in many industries such as the automotive, appliance, construction and furniture industries.
The major benefit that the use of coated steel gives is its improved corrosion resistance. There are many different types of corrosion in low carbon steel. It can be localized corrosion due to certain conditions in the environment or the corrosion can spread over the entire surface of the steel. Uniform corrosion is a form of corrosion that is uniformly spread over the steel and is clearly visible.
In addition there are several other forms of corrosion of low carbon steel including crevice corrosion, poultice corrosion, erosive corrosion, fretting corrosion. The need for corrosive resistant steel increased to cope with a wide variety of corrosive factors present in daily living conditions. With coated steel technology advancing fast, there are many different options available to meet the needs of various different industries.
Hot-dip galvanized steel is primarily used for applications such as automobile parts, air conditioning equipment (ducting/culvert), electric panels, building structures (corrugated roofing/wall panels), home appliance, ceiling frames and many more. Its anti-corrosion ability generally depends on the quantity of zinc coated on the surface. G90 or less in normally used for indoor facilities or equipments while G90 or more is use in suburban districts. Painted G90 is used in areas within cities or in industrial zones as well as along the seashore due to the high density of salty atmosphere.
Galvannealed steel is used for automobile body panels including vehicles such as buses, trucks and cars. It is also used for parts that require a long lifespan.
Galvalume and Galfan steel has a longer lifespan than hot-dip galvanized steel and is therefore preferred in applications that requires a long term corrosion resistance. Both types are also used in applications with temperatures higher than 500 degree Fahrenheit because hot-dip galvanized steel is not able to withstand such high temperatures. Galvalume and Galfan applications includes industrial roofing and siding, electric boxes and panels, furnace parts, heating devices and under-body parts of automobile as well as many more.
Electro-galvanized steel can be used in home appliances that require a very smooth surface appearance. This includes DVD, computer and electronic equipment bodies.
Pre-painted steel is widely used in roofing and siding, electronic devices, exposed body parts of automobiles, household appliances, furniture, garage doors, indoor wall panels and containers. The anti-corrosion service life of the pre-painted steel depends on the surface paint used. If a polyester (PE) paint is used the service life is around 7 to 8 years. The service life of a product painted with silicone modified polyester (SMP) is around 10 to 12 years and 15 years for special modified polyester. Polyvinyl chloride painted products usually has a service life of between 15 and 20 years and between 20 and 25 years for polyvinylidene fluoride products.